1. FIELD OF THE INVENTION
This invention relates to composite spray coatings, and more particularly to a composite spray coating suitable for use in a sink roll and the like in a continuous hot-dip galvanization apparatus and having an excellent resistance to hot-dip galvanization.
The composite spray coating according to the invention is also useful as a member contacting molten zinc-aluminum alloy (bath-member) such as a cylinder, support roll, guide roll or the like for hot-dip zinc-aluminum alloy plating apparatus.
2. DESCRIPTION OF THE RELATED ART
This type of bath member is used by immersing in a hot-dip galvanization bath, or by arranging in a place which is liable to be adhered with scattered molten zinc, or in a place contacting with a high temperature galvanized steel sheet having hot dip adhered thereon.
Under such circumstances, the bath member is required to have the following properties:
(1) erosion due to hot-dip zinc hardly occurs; PA1 (2) wearing hardly occurs even in the contacting with a passing sheet (steel sheet); PA1 (3) peeling of adhered hot-dip zinc and maintenance and inspection are easy; PA1 (4) service life as a roll is long and cost is low; PA1 (5) it is durable to thermal shock when immersing in a high temperature hot-dip galvanization bath, and the like. PA1 (1) Al or Al--Zn alloy is sprayed onto a coating film of a non-oxide ceramic or a cermet of the ceramic and metal and then heated at a high temperature, e.g. 700.degree.-900.degree. C. to diffuse Al or Al--Zn alloy into the inside of the film; PA1 (2) the coating film of the non-oxide ceramic or the cermet thereof is immersed in a zinc plating bath containing Al (e.g. 5%) for about 2-3 days to concentrate Al into the film surface, which is used in an original zinc plating bath (usually containing 0.1-0.3% of Al); and PA1 (3) the coating film of non-oxide ceramic or the cermet thereof is embedded in powder consisting essentially of Al powder (e.g. 60% by weight of Al powder and 40% by weight of powder) or powder consisting essentially of Al--Zn alloy powder (e.g. 60% by weight of 50% Al-50% Zn alloy, 38% by weight of Al.sub.2 O.sub.3 and 2% by weight of NH.sub.4 Cl) and then heated at 500.degree.-800.degree. C. in an argon gas atmosphere for several hours.
As roll or bearing constituting parts for the plating usually used, there are, for example, bush, bearing, collar and end ball and the like formed by (1) spray-coating Co base self-fluxing alloy defined according to JIS H8303 (1976), (2) spray-coating a ceramic of ZrO.sub.2 and Al.sub.2 O.sub.3 as disclosed in Japanese Patent Laid Open Publication No. 61-117260, (3) mainly spray-coating a coating consisting of at least one of WC, Cr.sub.3 C.sub.2 and TiC and a hot corrosion-resistant metal of Ni, Si or the like or an oxide thereof and having a thickness of 0.1-2.4 mm as disclosed in Japanese Patent Laid Open Publication No. 58-37386, (4) forming Cr.sub.2 O.sub.3 onto a ceramic spray-coated coating mainly composed of a carbide by chemical densification process as disclosed in Japanese Utility Model Laid Open Publication No. 3-63565, and the like.
As seen from the above, there have been attempted (1) the improvement of coating material having excellent resistance to hot-dip galvanization, (2) the improvement of adhesion property of the coating, (3) the improvement of density of the coating, (4) the control of surface roughness of the coating and the like, whereby the coating formed on the surface of the bath members for the conventional hot dip galvanization have sufficiently improved.
However, it cannot be said that the above conventional techniques are satisfactorily responded to the requirement of developing the coatings having more improved resistance to hot-dip galvanization in accordance with the increase of operation ratio in the plating plant accomplished with the increase of recent demand of hot-dip galvanized steel sheets as well as the demand of improving the quality of the galvanized steel sheet itself.